This invention relates to a novel heat exchanger or quench cooler for quenching the effluent from a hydrocarbon cracking furnace. More particularly, the invention relates to the coupling between the cracking furnace tubes and the tubes of the quench cooler or transferline exchanger.
In the production of light olefins (ethylene, propylene, butadiene and butylenes) and associated aromatics (benzene, toluene, ethylbenzene, xylenes and styrene) by the thermal cracking of hydrocarbon feedstocks in the presence of steam, the cracking reactions are stopped by rapidly cooling or quenching the cracking furnace effluent. The quenching time is measured in milliseconds and has the purpose of "freezing" the furnace outlet composition at its momentary value to prevent degradation of the olefin yield through continuing secondary reactions. A number of different quench cooler designs are available in the marketplace depending upon the quantity of cracked gas to be cooled, the fouling tendencies of the furnace effluent and the pressure/temperature conditions of the steam to be generated. These designs range from conventional fixed tubesheet shell and tube heat exchangers to double pipe designs.
It is well known that for any given cracking furnace operating conditions, the yield of olefins can be maximized and quencher fouling minimized by decreasing the temperature of the gas leaving the cracking furnace as rapidly as possible. This requires that the quench cooler be positioned as close as possible to the cracking furnace outlet, that the volume of the inlet section of the quench cooler be minimized and that the surface to volume ratio in the cooling section be maximized. The latter requirement implies that a multiplicity of small quencher tubes are more favorable than a single large diameter arrangement.
One prior art type of quench cooler known as the SHG transferline exchanger (Schmidt'sche Heissdampf - Gesellschaft mbH) uses a multiplicity of double tube arrangements in parallel wherein each quench tube is surrounded by a concentric outer tube which carries the water-steam mixture. The annuli between the inner and outer tubes are supplied with boiler water through horizontal, oval-shaped headers. In this regard, see German Patentschrift DE 2551195. Another prior art patent which uses this double tube arrangement with an oval header for the outside tubes is U.S. Pat. No. 4,457,364. This patent discloses a distributor having an inlet for the gas from the furnace and two or three diverging branches forming a wye or tri-piece for the transition between the furnace and the quench cooler. As indicated, this transition where cooling has not yet begun can be critical in minimizing continued reaction and undesirable coke deposits. In this U.S. Pat. No. 4,457,364, the cross sectional area for flow through the connector is substantially uniform to achieve substantially constant gas velocity throughout the distributor. The distributor may also be divergent in cross sectional area up to the point where the ratio of the sum of the cross sectional areas of the branches to the cross sectional area of the inlet is 2:1.